1. Field of the Invention
The present invention relates to a method of filling conductive filler material into through holes of a printed wiring board.
2. Description of the Prior Art
Conventionally, in order to fill through holes of a printed wiring board with conductive filler material, this filler material is caused to adhere to the interior of through holes using a number of pins adhered with conductive filler material by inserting these pins externally bearing this material into through holes of a printed wiring board. Alternatively, there is another conventional method which initially prepares a silk screen compatible with the designed circuitry of the printed wiring board in order that the conductive filler material can be filled into through holes of the printed wiring board via the silk screen.
Nevertheless, when applying the former pin insertion method, the amount of conductive filler material adhering to the external surface of these pins is inadequate. According to this conventional method, the processing system requires repeatedly inserting the pins into each through hole for many rounds before eventually filling up each through hole with enough amount of conductive filler material. This is turn involves much inconvenience and poor operating efficiency.
Furthermore, when executing the above operation with a number of pins, the conductive filler material adhered to the interior of through holes must be cured every round. This is turn causes the substrate of the printed wiring board to quickly incur substantial damage caused by thermal effect, thus significantly degrading the dimensional stability of the printed wiring board itself.
Furthermore, if the printed wiring board were provided with a number of through holes, then a large number of pins must be used with every round of operation. Practically, no system can use more than 1,000 pins per round of operation. Since the number of workable pins is limited, there is a critical limit in the size of each printed wiring board. For example, normally, a size of more than 300 square millimeters cannot be obtained.
This is turn involves difficulty in the manufacture of pin jigs arranging pins at positions corresponding to the positions of the through holes of the printed wiring board. Additionally, this process results in poor workability for executing mass production of the printed wiring board.
On the other hand, when applying the latter conventional method based on the silk screening method, squeegee pressure may not evenly be applied to the silk screen. Even when applying normal squeegee pressure, it is still difficult for this conventional method to fully fill up all the through holes of a printed wiring board with the conductive filler material. For example, if the printed wiring board were too thick or the diameter of the through hole were too narrow, then the conductive filler material cannot be filled into these through holes at all. For example, under the technical skill available today, if the printed wiring board were provided with 1.6 mm of thickness, then a diameter of 0.7 mm of each through hole is the minimum critical limit, and thus, if the through hole had a diameter of less than 0.7 mm, then the conductive filler material cannot be filled into this through hole at all.
Furthermore, when applying either of these conventional methods, fluidity of the conductive filler material severely affects the uniformity of the filling effect, and thus, much complexity is present in the practice of any conventional system. Normally, the conventional methods add a solvent to the conductive filler material in order to increase the fluidity of the conductive filler material. However, when curing the conductive filler material filled in the through hole, solvent thermally vaporizes inside the through hole, thus generating bubble and crack as well. This in turn causes the through holes to clog. Once the through hole is clogged, it is difficult to remove residual solvent.
More particularly, it is conventional that high-grade alcoholic solvent having a high boil point is used in order to properly maintain fluidity of the conductive filler material. In order to fully remove vaporized solvent, it is essential for any system to properly adjust temperature and time so that no bubble can be generated. Perfect removal of solvent from the through hole promotes the reliability of the produced printed wiring board. The invention has been achieved to fully solve those defects inherent in the conventional methods of filling conductive filler material. The object of the invention is to provide a novel method of uniformly filling conductive filler material into through holes of a printed wiring board independent of the thickness of printed wiring board, and the number and the diameter of through hole.